As a passive optical access technology using a point-to-multipoint topology structure, a Gigabit-Passive Optical Network (GPON) includes an Optical Line Terminal (OLT) installed in a central control station, a batch of Optical Network Units (ONUs) on subscriber sites, and an Optical Distribution Network (ODN). The OLT provides an interface between a network side and a core network for an access network, and is connected to the ONUs through the ODN. The ONUs provide interfaces of a subscriber side for the access network, so that various service streams such as voice, data and video are connected to the OLT through the ODN and are under centralized control of the OLT.
The telecommunication network is in a process of transiting towards packet-based transmission, but a service of transmitting a constant rate customer data stream is still an important profit-generating part of an operator, and the service of bearing the constant rate customer data stream through the GPON will exist for a long time. An existing procedure for transmitting the constant rate data stream (for example, a Time Division Multiple (TDM) data stream) through the GPON network includes the following steps:
In S1, the OLT (or the ONU) encapsulates the TDM data stream by using a variable-length GPON Encapsulation Method (GEM). The TDM data stream flows through a TDM buffer and is then encapsulated into a GEM frame. When an output rate and an input rate of the TDM buffer have a frequency offset, the length of the GEM frame varies with the frequency offset. A payload length indicator (PLI) field in a GEM frame header is used to indicate a payload length of the GEM frame, and the payload length is the number of valid bytes of the TDM data encapsulated by the GEM frame.
In S2, through GPON Transmission Convergence Layer (GTC) processing, the OLT (or the ONU) transmits the GEM frame to the ONU (or the OLT) through the GPON network.
In S3, the ONU (or the OLT) de-encapsulates the GTC frame to obtain the GEM frame, de-encapsulates the GEM frame to obtain the TDM data stream, puts the TDM data stream to a buffer, controls outputting of the TDM data stream by using a local clock, and adjusts an output frequency by determining a data amount in the buffer. If finding that the data amount in the buffer is continuously increased, the ONU increases the output frequency; and if finding that the data amount in the buffer is continuously decreased, the ONU decreases the output frequency, so as to ensure that the data is not lost.
In the implementation of the present invention, the inventor found that the prior art at least has the following problems:
Due to the GTC transmission system and Dynamic Bandwidth Assignment (DBA) technology in the GPON network, the arrival time of the GEM frame received by the ONU (or the OLT) is non-uniform, and the length change of the valid payload is great, so that the rate of outputting the data stream from the ONU (or the OLT) is inconsistent with the constant rate of inputting the TDM data to the OLT (or the ONU), that is, the data stream output from the ONU (or the OLT) cannot be consistent with the constant rate of inputting the constant rate data stream to the OLT (or the ONU) with a high quality. Specific reasons are described in the following.
In one aspect, because the TDM data stream is asynchronous with the GPON network, the TDM data stream can be transmitted in the GPON network before the data stream is encapsulated by the GEM frame. Because the output rate and the input rate of the TDM buffer have the frequency offset, when the GEM frame uses a fixed payload length for encapsulation, the sending rate of the GEM frame is different, and as a result, the rate of the GEM frame received by the ONU (or the OLT) is different; and when the GEM frame is sent by using a fixed rate, the payload length in the GEM frame is changed, the OLT (or the ONU) performs the encapsulation in bytes, and a minimum variation is 1 byte, and as a result, the length change of the valid payload of the GEM frame received by the ONU (or the OLT) is also at least 1 byte.
In other aspect, when the bandwidth utilization is improved by using the DBA technology, the time slot of the GEM frame in the GTC is not fixed, and the difference may approach one GTC frame, that is, 125 microseconds (μs), so that the arrival time of the GEM frame in the ONU (or the OLT) becomes more non-uniform.
Because the rate of the GEM frame received by the ONU (or the OLT) is non-uniform, and the length change of the valid payload is great, the ONU (or the OLT) requires a large buffer to adjust the output frequency of the TDM data stream, which increases the time delay of the TDM data stream.